Poppet valves are widely used for opening and closing flow paths for gases and liquids, for example, as shift diverter valves in vehicles; for channeling hydrogen into reformers in fuel cells; and as intake, exhaust, and exhaust gas recirculation valves in internal combustion engines. A poppet valve typically comprises a circular valve head mounted transversely on a pintle shaft for axial motion thereof to form or eliminate a flow gap between the valve head and a mating circular valve seat in a valve body. A pintle shaft is also known as a valve shaft or valve stem.
Poppet valves can be very demanding in their design and fabrication tolerances. For example, some poppet valves are provided with conical mating surfaces on the valve head and valve seat; however, leak-free sealing over the entire mating surfaces depends on near-perfect circularity, coaxiality, and smoothness of both surfaces. In addition, the surfaces must have substantially identical cone angles. The highest quality valves are ground on a precision lathe and then the valve head is lap-ground to its dedicated seat. This is labor-intensive, time-consuming, and expensive. U.S. Pat. No. 4,565,217 discloses a poppet valve having a conical head and a curved seat such that head and seat can seal along a circular line of contact, thereby obviating the need for matching cone angles.
Known poppet valves further require that the valve head be mounted perfectly orthogonal to the valve stem and that a plane containing the valve seat be perfectly parallel to a plane containing the valve head. Deviation in any of these relationships can cause the head to mate imperfectly with the seat, which can result in valve leakage.
Poppet valves used to divert a portion of an engine's exhaust gas stream into the intake stream (Exhaust Gas Recirculation, or EGR) can encounter significantly different pressures in these two streams, such that a single poppet control valve between them may require a powerful actuator to overcome the pressure difference. It is known to provide, therefore, a force-balanced poppet valve, having two valve elements operated by a single pintle shaft, one valve opening into the exhaust stream and the other opening into the intake stream, wherein the intake vacuum and exhaust pressure exerted across the two valves substantially cancel each other, permitting the valve to be controlled by an inexpensive, low-power actuator.
An important performance criterion for such a force-balanced valve is that the two valves open and close to a high degree of synchronicity. This requires that the spacing of the valve heads on the pintle shaft be identical with the spacing of the valve seats in the valve body. It requires further that the identical spacing be maintained over the full range of operating conditions, particularly temperatures.
The latter behavior can be provided by constructing the valve body, valve heads, and pintle shaft of the same material, for example, stainless steel, such that these components expand and contract at the same rate in response to thermal changes.
The requirement for identical spacing of the heads is more difficult to satisfy. Typically, during assembly, the first head is installed on the pintle shaft, either by being formed with the shaft or by being attached thereto, and seated on its seat; then the second head is slid onto the pintle shaft, seated against its own seat, and secured to the shaft as by welding, bolting, or the like. A problem with these techniques is that the second head and/or shaft may be deformed thereby, ruining the synchronicity of the assembled valve. If the head or shaft is distorted so that the head is not perfectly orthogonal to the shaft, the resulting valve cannot close properly. Such deformation can also create error in the axial location of the distal end of the pintle shaft, which may be cooperative with a position sensing apparatus during throttling operation of the valve. Valves thus deformed, if still usable, may require individual calibration after assembly, which is costly and time-consuming.
Alternatively, the shaft may be provided with an accurately-located inner shoulder against which the second valve head may be seated in the proper relationship to the first valve head and then be secured as described supra. Such a configuration is highly intolerant of error in placement of the shoulder or error during assembly, and is also vulnerable to the deformation potential noted above.
What is needed is a two-headed poppet valve wherein the spacing of the valve heads along the pintle shaft is directed by the spacing between the valve seats, wherein at least one of the valve heads is compliantly mounted on the pintle shaft, and wherein the compliantly-mounted head may be so mounted without engendering errors in spacing of the valve seats along the pintle shaft.